Distance sensor and image processing system including the same

ABSTRACT

A pixel of a distance sensor includes a photosensor that generates photocharges corresponding to light incident in a first direction. The photosensor includes a plurality of first layers having a cross-sectional area increasing along the first direction after a first depth and at least one transfer gate which receives a transfer control signal for transferring the photocharges to a floating diffusion node. A strong electric field is formed in the direction in which the photocharges move horizontally or vertically in the pixel, thereby accelerating the photocharges, allowing for increased sensitivity and demodulation contrast.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 14/075,174, filed Nov. 8, 2013, the entire contents of whichare incorporated herein by reference in their entirety.

BACKGROUND

Exemplary embodiments in accordance with principles of inventiveconcepts relate to a pixel, or pixel sensor, included in a depth sensor(also referred to herein as a distance sensor) and an image processingsystem including the same, and more particularly, to a pixel included ina distance sensor for providing high sensitivity and demodulationcontrast and an image processing system including the same.

Image sensors include a plurality of pixels, also referred to herein aspixel sensors, that convert photons within a spectrum band intoelectrons. Image sensors are widely employed in portable electronicdevices, such as cellular telephones or, or, more particularly, smartphones, and improvements in such sensors would benefit a large number ofusers.

Information about the distance (also referred to herein as depth)between an object and an image sensor, as well as about the color of animaged object is necessary to obtain a three-dimensional (3D) image.Methods for obtaining information about the distance between the objectand the image sensor may be active or passive.

Time-of-flight (TOF) and a triangulation methods are representativeactive methods. In the TOF method, modulated light is emitted to theobject, light reflected from the object is sensed, and the distance iscalculated from a phase change in the reflected light. In thetriangulation method, the position of light emitted by a laser or thelike a certain distance from the sensor and the position of reflectedlight are sensed and the distance between the sensor and the object iscalculated using triangulation. In passive methods, the distance iscalculated using only image information, without emitting light. Astereo camera representatively uses a passive method.

SUMMARY

In exemplary embodiments in accordance with principles of inventiveconcepts, a pixel of a distance sensor, includes: a photosensor arrangedto generate photocharges corresponding to light incident in a firstdirection and to have a cross-sectional area increasing along the firstdirection after a first depth; and at least one transfer gate arrangedto receive a transfer control signal for transferring the photochargesto a floating diffusion node.

In exemplary embodiments in accordance with principles of inventiveconcepts, the photosensor comprises a plurality of first layers having apinning voltage that increases along the first direction.

In exemplary embodiments in accordance with principles of inventiveconcepts, a doping density is arranged to determine the pinning voltageof the first layers.

In exemplary embodiments in accordance with principles of inventiveconcepts, the cross-sectional area of the photosensor decreases alongthe first direction up to the first depth.

In exemplary embodiments in accordance with principles of inventiveconcepts, the photosensor comprises a plurality of second layers havinga cross-sectional area that increases along a second direction and thesecond direction gets towards the floating diffusion node inperpendicular to the first direction.

In exemplary embodiments in accordance with principles of inventiveconcepts, the second layers have a pinning voltage that increases alongthe second direction.

In exemplary embodiments in accordance with principles of inventiveconcepts, the pinning voltage of the second layers is determined by oneof a doping density and a junction depth.

In exemplary embodiments in accordance with principles of inventiveconcepts, the at least one transfer gate comprises two transfer gates ina 2-tap structure.

In exemplary embodiments in accordance with principles of inventiveconcepts, a pixel further comprises a pinning layer between the twotransfer gates.

In exemplary embodiments in accordance with principles of inventiveconcepts, two transfer gates respectively receive transfer controlsignals respectively having opposite phases.

In exemplary embodiments in accordance with principles of inventiveconcepts, a pixel further comprises at least one select gate arranged toreceive a selection control signal for outputting an electrical signalcorresponding to photocharges accumulated in the floating diffusionnode.

In exemplary embodiments in accordance with principles of inventiveconcepts, an image processing system includes: a distance sensorcomprising a plurality of pixels each generating photochargescorresponding to light incident along a first direction and outputtingan electrical signal corresponding to the photocharges; and a processorarranged to calculate a distance to an object by processing electricalsignals using time-of-flight (TOF) method, wherein each of the pixelscomprises: a photosensor arranged to have a cross-sectional areaincreasing along the first direction after a first depth; and at leastone transfer gate arranged to receive a transfer control signal fortransferring the photocharges to a floating diffusion node.

In exemplary embodiments in accordance with principles of inventiveconcepts a photosensor comprises a plurality of first layers having apinning voltage that increases along the first direction.

In exemplary embodiments in accordance with principles of inventiveconcepts, in an image processing system, the cross-sectional area of thephotosensor decreases along the first direction up to the first depth.

In exemplary embodiments in accordance with principles of inventiveconcepts, in an image processing system the photosensor comprises aplurality of second layers having a cross-sectional area that increasesalong a second direction and the second direction gets towards thefloating diffusion node in perpendicular to the first direction.

In exemplary embodiments in accordance with principles of inventiveconcepts, an electronic device, includes: a pixel of a distance sensorthat includes a photosensor that generates photocharges corresponding tothe intensity of light impinging on a surface of the photosensor thephotosensor including layers of increasing cross sectional area, after afirst depth, in a direction parallel to the surface of the photosensorupon which light impinges; a floating diffusion node; and a transfergate arranged to transfer photocharges from the photosensor to afloating diffusion node.

In exemplary embodiments in accordance with principles of inventiveconcepts, in an electronic device of the increased cross sectional areasare arranged to accelerate photocharges.

In exemplary embodiments in accordance with principles of inventiveconcepts, an electronic device further comprises a plurality of thepixels of a distance sensor; a light source; and a processor arranged tocalculate a distance to an object by processing electrical signals usingtime-of-flight (TOF) method and using light from the light source andreflected from the object to the pixels.

In exemplary embodiments in accordance with principles of inventiveconcepts, an electronic device is a portable electronic device furthercomprising a processor.

In exemplary embodiments in accordance with principles of inventiveconcepts, a portable electronic device is a cellular telephone.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of inventive concepts willbecome more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a block diagram of a distance sensor according to exemplaryembodiments in accordance with principles of inventive concepts;

FIG. 2 is a circuit diagram of a 2-tap distance pixel illustrated inFIG. 1;

FIG. 3 is a timing chart showing the operation of the 2-tap distancepixel illustrated in FIG. 2;

FIG. 4 is a diagram of the layout of the distance pixel illustrated inFIG. 1;

FIG. 5 is a diagram showing the change in potential of a part of thelayout illustrated in FIG. 4;

FIG. 6 is a diagram of the cross-section of a part of the layoutillustrated in FIG. 4;

FIG. 7 is a diagram showing the change in potential of the cross-sectionillustrated in FIG. 6;

FIG. 8 is a block diagram of an image processing system including thedistance sensor illustrated in FIG. 1 according to exemplary embodimentsin accordance with principles of inventive concepts; and

FIG. 9 is a block diagram of an electronic system including the distancesensor illustrated in FIG. 1 according to exemplary embodiments inaccordance with principles of inventive concepts.

DESCRIPTION

Various exemplary embodiments will be described more fully hereinafterwith reference to the accompanying drawings, in which exemplaryembodiments are shown. Exemplary embodiments may, however, be embodiedin many different forms and should not be construed as limited toexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough, andwill convey the scope of exemplary embodiments to those skilled in theart. In the drawings, the sizes and relative sizes of layers and regionsmay be exaggerated for clarity.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to” or “coupled to” another element or layer, itcan be directly on, connected or coupled to the other element or layeror intervening elements or layers may be present. In contrast, when anelement is referred to as being “directly on,” “directly connected to”or “directly coupled to” another element or layer, there are nointervening elements or layers present. Like numerals refer to likeelements throughout. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items. The term“or” is used in an inclusive sense unless otherwise indicated.

It will be understood that, although the terms first, second, third, forexample. may be used herein to describe various elements, components,regions, layers and/or sections, these elements, components, regions,layers and/or sections should not be limited by these terms. These termsare only used to distinguish one element, component, region, layer orsection from another region, layer or section. Thus, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of exemplary embodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

The terminology used herein is for the purpose of describing particularexemplary embodiments only and is not intended to be limiting ofexemplary embodiments. As used herein, the singular forms “a,” “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Exemplary embodiments are described herein with reference tocross-sectional illustrations that are schematic illustrations ofidealized exemplary embodiments (and intermediate structures). As such,variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, exemplary embodiments should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the actualshape of a region of a device and are not intended to limit the scope ofexemplary embodiments.

Unless otherwise defined, all terms (including technical and scientificWI is) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which exemplary embodiments belong. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Hereinafter, exemplary embodiments in accordance with principles ofinventive concepts will be explained in detail with reference to theaccompanying drawings. FIG. 1 is a block diagram of an exemplaryembodiment of a depth, or distance, sensor 10 in accordance withprinciples of inventive concepts. FIG. 2 is a circuit diagram of a 2-tapdistance pixel 23 illustrated in FIG. 1. FIG. 3 is a timing chartshowing the operation of the 2-tap distance pixel 23 illustrated in FIG.2.

Referring to FIGS. 1 through 3, the distance sensor 10, which measures adistance or a depth using a time-of-flight (TOF) principle, includes asemiconductor chip 20 including an array 22 in which a plurality ofdistance pixels (that is, distance or depth detectors or sensors) 23 arearranged, a light source 32, and a lens module 34. The pixels 23 may beimplemented in a 2-tap structure as shown in FIG. 2, or in a 1-tap or4-tap structure, for example. The distance sensor 10 may be implementedas a part of an image sensor (not shown) that generates athree-dimensional (3D) image using color and distance image information.

Distance pixels 23 implemented in the array 22 in two dimensions, eachinclude at least one transfer gate (for example, TX1 and TX2 in FIG. 2).Each of the distance pixels 23 includes a plurality of transistors forsignal processing.

Row decoder 24 selects one row from among a plurality of rows inresponse to a row address output from a timing controller 26. In thisexemplary embodiment in accordance with principles of inventiveconcepts, a row is a set of distance pixels arranged in a lineardirection, a horizontal direction, for example, in the array 22.

In an exemplary embodiment in accordance with principles of inventiveconcepts, photo gate controller 28 may generate transfer control signalsTS1 and TS2 and provide them to the array 22 under the control of thetiming controller 26. The transfer control signals TS1 and TS2 may beone of first through fourth photo gate control signals Ga through Gdillustrated in FIG. 3. The photo gate controller 28 may generate aplurality of control signals RS1, RS2, SEL1, and SEL2 applied to thedistance pixels 23 under the control of the timing controller 26.

As shown in FIG. 3, in exemplary embodiments in accordance withprinciples of inventive concepts, the difference between the phase ofthe first photo gate control signal Ga and the phase of the third photogate control signal Gc is 90°, the difference between the phase of thefirst photo gate control signal Ga and the phase of the second photogate control signal Gb is 180°, and the difference between the phase ofthe first photo gate control signal Ga and the phase of the fourth photogate control signal Gd is 270°.

In exemplary embodiments in accordance with principles of inventiveconcepts, light source driver 30 may generate a clock signal MLS fordriving the light source 32 under the control of the timing controller26. In exemplary embodiments in accordance with principles of inventiveconcepts, light source 32 emits a modulated optical signal EL to a scene40 in response to the clock signal MLS. The modulated optical signal ELmay have amplitude varying according to the driving of the light sourcedriver 30. In operation, the distance between the scene 40 and thedistance sensor 10 may vary. In exemplary embodiments in accordance withprinciples of inventive concepts, a light emitting diode (LED), anorganic LED (OLED), an active-matrix OLED (AMOLED), or a laser diode maybe used as the light source 32 and the clock signal MLS or the modulatedoptical signal EL may be a sine wave or a square wave, for example.

The light source driver 30 applies the clock signal MLS or informationabout the clock signal MLS to the photo gate controller 28. Photo gatecontroller 28 generates the first photo gate control signal Ga havingthe same phase as the clock signal MLS and the second photo gate controlsignal Gb whose phase is different by 180° from the phase of the clocksignal MLS. The photo gate controller 28 also generates the third photogate control signal Gc whose phase is different by 90° from the phase ofthe clock signal MLS and the fourth photo gate control signal Gd whosephase is different by 270° from the phase of the clock signal MLS. Thephoto gate controller 28 and the light source driver 30 may operate insynchronization with each other, for example.

In operation, the modulated optical signal EL output from the lightsource 32 is reflected from the scene 40. When there are differentdistances Z₁, Z₂, and Z₃ between the distance sensor 10 and the scene40, a distance Z is calculated as follows. When, for example, themodulated optical signal EL is represented as a waveform: A_(in)cos(ωt), where the waveform period T is given by T=2π/ω, A_(in) is theamplitude, and an optical signal RL reflected to a distance pixel isrepresented as a waveform: A_(ref) cos(ωt+θ), where A_(ref) is theamplitude, a phase shift or phase difference “θ” by TOF is defined asEquation 1:θ=2*ω*Z/C=2*(2πf)*Z/C,  (1)

where C is the speed of light and f is the frequency of the signal.

Accordingly, the distance Z from the light source 32 or the array 22 tothe target scene 40 is calculated using Equation 2:Z=θ*C/(2*ω)=θ*C/(2*(2πf)).  (2)

The reflected optical signal RL is incident on the array 22 through thelens module 34. In this exemplary embodiment in accordance withprinciples of inventive concepts, the lens module 34 may include a lensand an infrared pass filter and distance sensor 10 includes a pluralityof light sources arranged in circle around the lens module 34, but onlyone light source 32 is illustrated in FIG. 1 for clarity of thedescription.

The reflected optical signal RL incident on the array 22 through thelens module 34 may be demodulated through N times of sampling, (that is,N samplings. For example, 2 times of sampling for a 2-tap distancepixel). The term sampling indicates that a pixel signal A0, A1, A2, orA3 is generated or detected from the reflected optical signal RL. Thepixel signal A0, A1, A2, or A3 will be described in greater detailbelow. The phase shift θ between the modulated optical signal EL and thereflected optical signal RL may be expressed by Equation 3:

$\begin{matrix}{\theta = {- {{\arctan\left( \frac{{A\; 3} - {A\; 1}}{{A\; 2} - {A\; 0}} \right)}.}}} & (3)\end{matrix}$

An amplitude A of the reflected optical signal RL may be expressed byEquation 4:

$\begin{matrix}{A = {\frac{\sqrt{\left( {{A\; 3} - {A\; 1}} \right)^{2} + \left( {{A\; 0} - {A\; 2}} \right)^{2}}}{2}.}} & (4)\end{matrix}$

The amplitude A of the reflected optical signal RL is determined byamplitude of the modulated optical signal EL. An offset B of thereflected optical signal RL may be expressed by Equation 5:

$\begin{matrix}{B = {\frac{{A\; 0} + {A\; 1} + {A\; 2} + {A\; 3}}{4}.}} & (5)\end{matrix}$

In exemplary embodiments in accordance with principles of inventiveconcepts, each of the distance pixels 23 includes a photo-receiver, suchas a photodiode PD, two transfer transistors TX1 and TX2, two resettransistors RX1 and RX2, two drive transistors DX1 and DX2, and twoselect transistors SX1 and SX2.

In exemplary embodiments in accordance with principles of inventiveconcepts, a distance pixel 23 having the 2-tap structure including onephotodiode and eight metal oxide semiconductor (MOS) transistors isillustrated in FIG. 2, but the inventive concept is not restricted tothe embodiments illustrated in FIG. 2 and may be applied to otherconfigurations, such as 1-tap or 4-tap distance pixels. In otherembodiments, at least one of the transistors RX1, RX2, DX1, DX2, SX1,and SX2 may be omitted.

In operation in accordance with principles of inventive concepts ofdistance pixel 23, the photodiode PD generates photocharge that varieswith the intensity of the reflected optical signal RL. The firsttransfer transistor TX1 may transfer the photocharge to a first floatingdiffusion node FD1 in response to the first transfer control signal TS1output from the photo gate controller 28. The second transfer transistorTX2 may transfer the photocharge to a second floating diffusion node FD2in response to the second transfer control signal TS2 output from thephoto gate controller 28. In exemplary embodiments in accordance withprinciples of inventive concepts, while the first and second transfercontrol signals TS1 and TS2 are at a high level (of, for example, 3.3V), the first and second transfer transistors TX1 and TX2 transfer thephotocharge generated by a photodetector, such as photodiode PD, to thefloating diffusion nodes FD1 and FD2, respectively and while the firstand second transfer control signals TS1 and TS2 are at a low level (of,for example, 0 V), the first and second transfer transistors TX1 and TX2do not transfer the photocharge to the floating diffusion nodes FD1 andFD2.

In exemplary embodiments in accordance with principles of inventiveconcepts, during a first sampling period Sample1, the first photo gatecontrol signal Ga and the second photo gate control signal Gb may beprovided as the first transfer control signal TS1 and the secondtransfer control signal TS2, respectively. During a second samplingperiod Sample2, the third photo gate control signal Gc and the fourthphoto gate control signal Gd may be provided as the first transfercontrol signal TS1 and the second transfer control signal TS2,respectively.

In exemplary embodiments in accordance with principles of inventiveconcepts, each of the first and second sampling periods Sample1 andSample2 may corresponding to a single period of the clock signal MLS andtwo sampling periods are employed to generate distance information for asingle 2-tap distance pixel 23. When a modulation frequency is 20 MHz,the first and second sampling periods Sample1 and Sample2 each are 50nsec and the first and second sampling periods Sample1 and Sample2 maycome consecutively or intermittently.

In other exemplary embodiments in accordance with principles ofinventive concepts, four sampling periods may be used to generatedistance information for a single 1-tap distance pixel and one samplingperiod may be used to generate distance information for a single 4-tapdistance pixel.

In exemplary embodiments in accordance with principles of inventiveconcepts, the first drive transistor DX1 may amplify and transmit thephotocharge to the first select transistor SX1 according to thepotential of the photocharge accumulated in the floating diffusion nodeFD1. The second drive transistor DX2 may amplify and transmit thephotocharge to the second select transistor SX2 according to thepotential of the photocharge accumulated in the floating diffusion nodeFD2.

In exemplary embodiments in accordance with principles of inventiveconcepts, the first select transistor SX1 may have a drain terminalconnected to a source terminal of the first drive transistor DX1 and mayoutput a pixel signal to a correlated double sampling(CDS)/analog-to-digital conversion (ADC) circuit 36 through a firstcolumn line COL1 in response to the selection control signal SEL1 outputfrom the photo gate controller 28. The second select transistor SX2 mayhave a drain terminal connected to a source terminal of the second drivetransistor DX2 and may output a pixel signal to the CDS/ADC circuit 36through a second column line COL2 in response to the selection controlsignal SEL2 output from the photo gate controller 28.

The first reset transistor RX1 may reset the floating diffusion node FD1in response to the reset control signal RS1 output from the photo gatecontroller 28. The second reset transistor RX2 may reset the floatingdiffusion node FD2 in response to the reset control signal RS2 outputfrom the photo gate controller 28.

In exemplary embodiments in accordance with principles of inventiveconcepts, the distance pixel 23 accumulates photocharge for apredetermined period of time, which may be referred to as, for example,an integration time, and outputs pixel signals A0, A1, A2, and A3generated as a result of the accumulation. A pixel signal A_(k)generated by the distance pixel 23 is expressed by Equation 6:

$\begin{matrix}{A_{k} = {\sum\limits_{n = 1}^{N}{a_{k,n}.}}} & (6)\end{matrix}$

In exemplary embodiments in accordance with principles of inventiveconcepts, when a signal input to the gates (TG1 and TG2 in FIG. 4) ofthe respective transfer transistors TX1 and TX2 of the distance pixel 23is the first photo gate control signal Ga, “k” is 0; when it is thethird photo gate control signal Gc, “k” is 1; when it is the secondphoto gate control signal Gb, “k” is 2; and when it is the fourth photogate control signal Gd, “k” is 3. In Equation 6, a_(k,n) indicates thenumber of photons (or photocharges) generated in the distance pixel 23when an n-th (“n” is a natural number) gate signal is applied with aphase difference corresponding to “k” and N=fm*Tint where “fm” denotesthe modulation frequency and “Tint” denotes the integration time (forexample, a time for which photocharges are accumulated in a floatingdiffusion node during a single sampling period).

Referring to FIG. 1, under the control of the timing controller 26, theCDS/ADC circuit 36 performs correlated double sampling (CDS) and analogto digital converting (ADC) on the pixel signals A0, A1, A2, and A3output from the distance pixel 23 and outputs digital pixel signals. Thedistance sensor 10 illustrated in FIG. 1 may also include active loadcircuits (not shown) to transmit pixel signals output from a pluralityof column lines implemented in the array 22 to the CDS/ADC circuit 36. Amemory 38, which s may be implemented as a buffer, receives and storesthe digital pixel signals output from the CDS/ADC circuit 36.

The distance sensor 10 may also include an image signal processor (ISP)39. The ISP 39 may process the pixel signals A0, A1, A2, and A3 outputfrom the memory 38 and compute distance, or depth, information.

FIG. 4 is a diagram of an exemplary embodiment of a layout 23-1 of eachof the distance pixels 23 illustrated in FIG. 1. FIG. 5 is a diagramshowing the change in the potential of a part of the layout 23-1illustrated in FIG. 4. Referring to FIGS. 1 through 5, the distancepixel layout 23-1 includes a shallow trench isolation (STI) region 42, awell region 44, the floating diffusion nodes FD1 and FD2, the gates TG1and TG2 of the respective transfer transistors TX1 and TX2, gates DG1and DG2 of the respective drive transistors DX1 and DX2, gates SG1 andSG2 of the respective select transistors SX1 and SX2, gates RG1 and RG2of the respective reset transistors RX1 and RX2, a photodiode 46, and apinning layer 48. Although FIG. 4 shows the layout of a 2-tap distancepixel, inventive concepts are not limited thereto and may be applied topixels having other structures such as 1-tap pixels and 4-tap pixels,for example. Additionally, the arrangement of elements is not restrictedto the exemplary embodiment of a distance pixel layout 23-1 inaccordance with principles of inventive concepts illustrated in FIG. 4.

In exemplary embodiments in accordance with principles of inventiveconcepts, the STI region 42 may be formed to electrically or opticallyisolate adjacent elements from each other. The STI region 42 formedusing an STI process may be filled with oxide and poly silicon, forexample. The STI region 42 may reduce or prevent electric crosstalk thatcould otherwise diminish signal-to-noise ratio (SNR) due to the exchangeof carriers between adjacent elements. The sidewall of the STI region 42may be doped with a material having a high reflectance, thereby reducingor preventing optical crosstalk that could otherwise diminish SNR due tothe permeation to a second distance pixel 23 of light that is incidenton a first distance pixel 23. In exemplary embodiments in accordancewith principles of inventive concepts, the sidewall of the STI region 42may be formed of poly silicon doped with boron having a highreflectance, for example.

The well region 44 may be formed around the gates RG1, RG2, DG1, DG2,SG1, and SG2. The well region 44 may include a high-density doped region(not shown). The high-density doped region may function as a source ordrain terminal of the transistors RX1, RX2, DX1, DX2, SX1, and SX2. Thewell region 44 may electrically insulate the high-density doped region.

The floating diffusion nodes FD1 and FD2, the gates TG1 and TG2 of therespective transfer transistors TX1 and TX2, the gates DG1 and DG2 ofthe respective drive transistors DX1 and DX2, the gates SG1 and SG2 ofthe respective select transistors SX1 and SX2, and gates RG1 and RG2 ofthe respective reset transistors RX1 and RX2 may accumulate photochargesor receive one of the control signals TS1, TS2, SEL1, SEL2, RS1, RS2, asdescribed above in the discussion related to FIG. 2.

In exemplary embodiments in accordance with principles of inventiveconcepts, the photodiode 46 may be formed using ion implantation. Thephotodiode 46 may be formed of an n-type region when the transfertransistors TX1 and TX2 are p-type MOS (PMOS) transistors. Thephotodiode 46 may be formed of a p-type region when the transfertransistors TX1 and TX2 are n-type MOS (NMOS) transistors. The pinninglayer 48 may be formed to have a high potential to electrically insulatethe gates TG1 and TG2 from each other.

The photodiode 46 may include a plurality of doped regions in a stackstructure, for example. The photodiode 46 may include a plurality ofsecond layers 50 in a second direction. The second layers 50 may bedefined by a border at which a cross-sectional area or a pinning voltagechanges for clarity of the description. The cross-sectional area or thepinning voltage of the photodiode 46 may continuously vary.

The second layers 50 may have a cross-sectional area continuouslyincreasing along a second direction. As the cross-sectional area of thesecond layers 50 increases along the second direction, the potential ofthe second layers 50 decreases, so that a strong electric field isformed in the second direction.

In other exemplary embodiments in accordance with principles ofinventive concepts, the second layers 50 may have a fixedcross-sectional area intermittently increasing at borders L3, L4, L5 L8,L9, and L10. In addition, the second layers 50 may have a pinningvoltage increasing along the second direction. That is, in exemplaryembodiments in accordance with principles of inventive concepts, thesecond layers 50 may have a doping density or a junction distanceincreasing along the second direction. As a result, the potentialintermittently decreases at the borders L3, L4, L5 L8, L9, and L10 and astrong electric field is formed in the second direction.

In still other exemplary embodiments in accordance with principles ofinventive concepts, the photodiode 46 may be formed to have a dopingdensity or junction depth continuously increasing along the seconddirection.

FIG. 5 illustrates the potential distribution in a lateral directionL-L′ of a part 23-a of the distance pixel layout 23-1 when the firsttransfer control signal TS1 at the high level (for example, 3.3 V) isapplied to the gate TG1 of the first transfer transistor TX1 and thesecond transfer control signal TS2 at the low level (for example, 0 V)is applied to the gate TG2 of the second transfer transistor TX2. Thepotential is highest between the borders L6 and L7 in which the pinninglayer 48 is positioned, and intermittently decreases at the borders L6and L7 between the pinning layer 48 and the photodiode 46, at theborders L3, L4, L5, L8, L9, and L10 of the second layers 50, and at theborders L2 and L11 between the photodiode 46 and the first floatingdiffusion node FD1. Because the cross-sectional area of the secondlayers 50 continuously increases along the second direction, thepotential of the second layers 50 continuously decreases. The slope ofthe potential of the second layers 50 is determined by the rate at whichthe cross-sectional area of the second layers 50 increases. Accordingly,a strong electric field is formed in the second direction, increasingthe speed of photocharges.

However, the potential becomes high again from between the borders L8and L9 to between the borders L10 and L11, where the gate TG2 of thesecond transfer transistor TX2 to which the second transfer controlsignal TS2 at the low level (for example, 0 V) is applied is positioned.As a result, the photocharges are inhibited from movement in the seconddirection.

In exemplary embodiments in accordance with principles of inventiveconcepts, the sampling periods Sample1 and Sample2 illustrated in FIG. 3are very brief, and therefore, the photocharges generated in thephotodiode 46 need to move to the floating diffusion nodes FD1 and FD2as quickly as possible. According to exemplary embodiments in accordancewith principles of inventive concepts, a strong electric field is formedin the second direction, that is, a direction in which the photochargesmove horizontally, thereby accelerating the speed of the photocharges.As a result, the demodulation contrast of the distance pixel 23increases and, a pixel signal having high sensitivity can be obtainedwhile a fill factor, the ratio of a light receiving area to the area ofa distance pixel, and resolution are maintained.

Although eight second layers 50 are illustrated in the embodimentsillustrated in FIG. 4, inventive concepts are not restricted thereto.

FIG. 6 is a diagram of a cross-section 23-2 of a part of an exemplaryembodiment of a distance pixel layout 23-1 such as that illustrated inFIG. 4. FIG. 7 is a diagram showing the change in potential of thecross-section 23-2 illustrated in FIG. 6. Referring to FIGS. 1 through7, the cross-section 23-2 taken along the lateral direction L-L′ in FIG.5 includes the STI region 42, the floating diffusion nodes FD1 and FD2,the gates TG1 and TG2 of the respective transfer transistors TX1 andTX2, the photodiode 46, the pinning layer 48, and a gate insulationlayer 54.

Multiple layers of conductive lines (not shown) may be included in thetop of the cross-section 23-2 and the conductive lines may be formed,for example, by patterning a conductive material containing a metalmaterial such as copper or aluminum.

The photodiode 46 may include a plurality of first layers 56 formedalong a first direction, which may be a direction in which the reflectedoptical signal RL is incident on the distance pixel 23.

For brevity and clarity of description, the first layers 56 may bedefined by a border at which a cross-sectional area or a pinning voltagechanges, for example. In accordance with principles of inventiveconcepts, the cross-sectional area or the pinning voltage of thephotodiode 46 may continuously change.

The first layers 56 may have a cross-sectional area continuouslyincreasing along the first direction after a first depth. The firstdepth may be randomly determined along the vertical cross section of thephotodiode 46. In accordance with principles of inventive concepts, asthe cross-sectional area of the first layers 56 increases along thefirst direction, the potential of the first layers 56 decreases, so thata strong electric field is formed in the first direction. Thecross-sectional area of the first layers 56 may decrease (in a sectionbetween borders V3 and V5) or remain constant (in a section betweenborders V5 and V7) up to the first depth along the first direction inorder to increase the fill factor and the resolution.

In other exemplary embodiments in accordance with principles ofinventive concepts, the first layers 56 may have a fixed cross-sectionalarea after the first depth and intermittently increasing depth at aborder V2. In addition, the first layers 56 may be formed to have apinning voltage increasing along the first direction. That is, inaccordance with principles of inventive concepts, the first layers 56may have a doping density increasing along the first direction and, as aresult, the potential intermittently decreases at borders V2, V3, V4,V5, and V6 of the first layers 56 and a strong electric field is formedin the first direction. A first layer 56 (between the borders V1 and V2)at the top may include a plurality of the second layers 50 illustratedin FIG. 4. The potential of the second layers 50 may be lower than thatof a first layer 56 between the borders V2 and V3.

In other exemplary embodiments in accordance with principles ofinventive concepts, the photodiode 46 may have a doping densitycontinuously increasing along the first direction.

Referring to FIG. 7, the potential in a vertical direction V-V′ of thepart 23-a of the distance pixel layout 23-1 is highest between bordersV6 and V7 on which the reflected optical signal RL is first incident andintermittently decreases at the borders V6, V5, V4, V3, and V2 of thefirst layers 56. Because the cross-sectional area of the first layers 56continuously increases along the first direction after the first depth,the potential of the first layers 56 continuously decreases. The slopeof the potential of the first layers 56 is determined by a rate at whichthe cross-sectional area of the first layers 56 increases. Accordingly,in accordance with principles of inventive concepts, a strong electricfield is formed in the first direction, increasing the speed ofphotocharges.

The sampling periods Sample1 and Sample2 illustrated in FIG. 3 are verybrief, and therefore, the photocharges generated in the photodiode 46need to move to the floating diffusion nodes FD1 and FD2 as quickly aspossible. According to this exemplary embodiment in accordance withprinciples of inventive concepts, a strong electric field is formed inthe first direction, that is, a direction in which the photocharges movevertically, thereby accelerating the photocharges. Therefore, inaccordance with principles of inventive concepts, the demodulationcontrast of the distance pixel 23 increases and a pixel signal havinghigh sensitivity can be obtained while, at the same time, the fillfactor and the resolution are maintained constant.

Although six first layers 56 are illustrated in the exemplaryembodiments illustrated in FIG. 6, inventive concepts are not limitedthereto.

The gate insulation layer 54 may be formed below each of the gates TG1and TG2 of the respective transfer transistors TX1 and TX2. The gateinsulation layer 54 may be formed of SiO₂, SiON, SiN, Al₂O₃, Si₄N₄,Ge_(x)O_(y)N_(z), Ge_(x)Si_(y)O_(z), or a high dielectric material, forexample. The high dielectric material may be HfO₂, ZrO₂, Al₂O₃, Ta₂O₅,hafnium silicate, zirconium silicate, or a combination thereof which isformed using atomic layer deposition, for example.

FIG. 8 is a block diagram of an exemplary embodiment of an imageprocessing system 900 including the distance sensor 10 illustrated inFIG. 1 according to principles of inventive concepts. Referring to FIGS.1 and 8, the image processing system 900 may include the distance sensor10, a processor 910, a memory 920, a display unit 930, and an interface940.

The processor 910 may control the operation of the distance sensor 10 orprocess image data output from the distance sensor 10. For example, theprocessor 910 may generate two-dimensional (2D) distance image databased on distance information of the scene 40 output from the distancesensor 10. The processor 910 may be the ISP 39, for example.

The memory 920 may store a program for controlling the operation of thedistance sensor 10 through a bus 950 according to the control of theprocessor 910 and may store an image generated by the processor 910. Theprocessor 910 may access the memory 920 and execute the program. Thememory 920 may be implemented by non-volatile memory.

The display unit 930 may receive an image from the processor 910 or thememory 920 and display the image through a display such as a liquidcrystal display (LCD), a light emitting diode (LED) display, organic LED(OLED) display, an active matrix OLED (AMOLED) display, or a flexibledisplay. The interface 940 may be implemented to input and output 2D or3D images. The interface 940 may be a wireless interface, for example.

FIG. 9 is a block diagram of an exemplary embodiment of an electronicsystem 1000 in accordance with principles of inventive conceptsincluding a distance sensor 10 in accordance with principles ofinventive concepts, such as that illustrated in FIG. 1. Referring toFIGS. 1 and 9, the electronic system 1000 may be implemented as a dataprocessing device, such as a personal digital assistant (PDA), aportable media player (PMP), an Internet protocol television (IPTV), acellular telephone, a portable notebook or tablet computer, or a smartphone, which can use or support mobile industry processor interface(MIPI). The electronic system 1000 includes the distance sensor 10, anapplication processor 1010, and a display 1050.

A camera serial interface (CSI) host 1012 implemented in the applicationprocessor 1010 may perform serial communication with a CSI device 1041included in the distance sensor 10 through a CSI. In accordance withprinciples of inventive concepts, a deserializer DES and a serializerSER may be implemented in the CSI host 1012 and the CSI device 1041,respectively.

A display serial interface (DSI) host 1011 implemented in theapplication processor 1010 may perform serial communication with a DSIdevice 1051 included in the display 1050 through DSI. In accordance withprinciples of inventive concepts, a serializer SER and a deserializerDES may be implemented in the DSI host 1011 and the DSI device 1051,respectively.

The electronic system 1000 may also include a radio frequency (RF) chip1060 communicating with the application processor 1010. A physical layer(PHY) 1013 of the application processor 1010 and a PHY 1061 of the RFchip 1060 may communicate data with each other according to MIPI DigRF.

The electronic system 1000 may further include a global positioningsystem (GPS) 1020, a storage 1070, a microphone (MIC) 1080, a dynamicrandom access memory (DRAM) 1085, and a speaker 1090. The electronicsystem 1000 may communicate using a worldwide interoperability formicrowave access (Wimax) 1030, a wireless local area network (WLAN)1100, and/or an ultra-wideband (UWB) 1110.

As described above, in exemplary embodiments in accordance withprinciples of inventive concepts, a strong electric field is formed in adirection in which photocharges move horizontally or vertically in apixel of a distance sensor, thereby increasing the speed of thephotocharges and, as a result, sensitivity and demodulation contrast areincreased.

While inventive concepts have been shown and described with reference toexemplary embodiments thereof, it will be understood by those ofordinary skill in the art that various changes in forms and details maybe made therein without departing from the spirit and scope of inventiveconcepts as defined by the following claims.

What is claimed is:
 1. An image processing system comprising: a distancesensor comprising a plurality of pixels each generating photochargescorresponding to light incident along a first direction and outputtingan electrical signal corresponding to the photocharges; and a processorarranged to calculate a distance to an object by processing electricalsignals using time-of-flight (TOF) method, wherein each of the pixelscomprises: a photosensor arranged to have a cross-sectional areaincreasing along the first direction after a first depth; and at leastone transfer gate arranged to receive a transfer control signal fortransferring the photocharges to a floating diffusion node, wherein thecross-sectional area of the photosensor decreases along the firstdirection up to the first depth.
 2. The image processing system of claim1, wherein the photosensor comprises a plurality of first layers havinga pinning voltage that increases along the first direction.
 3. The imageprocessing system of claim 1, wherein the photosensor comprises aplurality of second layers having a cross-sectional area that increasesalong a second direction and the second direction gets towards thefloating diffusion node in perpendicular to the first direction.
 4. Anelectronic device, comprising: a pixel of a distance sensor thatincludes a photosensor that generates photocharges corresponding to theintensity of light impinging on a surface of the photosensor, thephotosensor including layers of increasing cross sectional area, after afirst depth, in a first direction parallel to the surface of thephotosensor upon which light impinges; a floating diffusion node; and atransfer gate arranged to transfer photocharges from the photosensor toa floating diffusion nod, wherein the cross-sectional area of thephotosensor decreases along a second direction perpendicular to thefirst direction up to the first depth.
 5. The electronic device of claim4, wherein the increased cross sectional areas are arranged toaccelerate photocharges.
 6. The electronic device of claim 4, furthercomprising: a plurality of the pixels of a distance sensor of claim 4; alight source; and a processor arranged to calculate a distance to anobject by processing electrical signals using time-of-flight (TOF)method and using light from the light source and reflected from theobject to the pixels.
 7. The electronic device of claim 6, wherein theelectronic device is a portable electronic device further comprising adisplay unit.
 8. The electronic device of claim 7, wherein the portableelectronic device is a cellular telephone.